This invention relates to a device for converting heat energy to mechanical energy.
In more detail the invention relates to a device that utilizes a shape-memory material to convert low temperature thermal energy to mechanical energy.
Sources of low-grade heat frequently exist at the site at which a demand exists for low-grade energy. For example, homes, farms, factories, office buildings, hotels, and the like frequently have a constant supply of hot water available, not all of which is utilized for its intended purpose. In addition, the waste heat of industrial, farm or home equipment may be economically utilized to heat water to a low temperature. Furthermore, it can confidently be predicted that low-grade heat in the form of hot water will in the near future be economically available almost everywhere from solar power. In general, this water is not hot enough to drive a turbine or use in any conventional type of energy-generating equipment.
Frequently, the energy demand may be for relatively low-power energy, either constant or constantly available even if not required constantly. Among the small-scale--and yet important--applications envisioned for heat engines working at low temperatures across a small .DELTA.T to produce a limited but significant amount of power are devices designed to orient solar apparatus, to regulate thermal heat control and as a thermal safety valve. Despite the limitations imposed on low-temperature heat engines, sources of low-grade heat are so wide spaced and available that utilization of a small fraction thereof could have a significant impact on the world energy supply.
Low-temperature energy conversion utilizing heat engines based on the energetic shape recovery of a deformed thermoelastic material--called a shape-memory material--have been extensively investigated. These "memory materials" are capable of undergoing a martensitic transition with the ability of the alloys to undergo such a transition being both temperature and stress dependent. The temperature range at which this transition can occur is hereinafter called the critical temperature and is characteristic of the particular alloy under consideration. At temperatures below their critical temperatures, these alloys are relatively soft and pliable. Annealed at a temperature above its critical temperature in a given shape (hereinafter designated the "heat-treated configuration") and deformed into a "trained configuration" at a temperature below that critical temperature, such an alloy will revert back to its trained configuration when heated to or above its critical temperature. The process is called shape-recovery since the alloy element will move in a direction opposite to the direction in which it had been deformed and in so doing will recover its trained shape. This recovery is found to occur with some force supplied by a part of the martensitic latent heat of transformation which is about 2 cal/g. Experimentally it is found that the portion of this thermal energy which can be converted to the mechanical energy is about 0.16, and the Nitinol engine efficiency cannot exceed 5%. These memory materials are produced in the forms of wires, rods, foils, plates, tubes, springs, etc., and with wide ranges of temperature responsiveness which is a function of alloy composition and production history. For example, memory materials are available that possess critical temperatures in the range of from -150.degree. C. to +150.degree. C. The alloys of one such series, referred to as 55-Nitinol, have chemical compositions in the range from about 53 to about 57 weight percent nickel, balance titanium, and are based on the intermetallic compound NiTi. Descriptions of these and other titanium-based memory materials are given in U.S. Pat. Nos. 3,174,851 (W. J. Buehler et al, "Nickel-Base Alloys," Mar. 23, 1965); 3,403,238 (W. J. Buehler et al, "Conversion of Heat Energy to Mechanical Energy," Sept. 24, 1968); and 3,558,369 (F. E. Wang et al, "Method of Treating Variable Transition Temperature Alloys," Jan. 26, 1971).
These alloys have found practical application in a number of different areas such as control devices and medical instruments. A wide variety of heat engines employing the alloys have also been proposed. A partial list of patents directed to such devices follows:
U.S. Pat. No. 3,913,326--Banks--Oct. 21, 1975 PA1 U.S. Pat. No. 4,027,479--Cory--May 6, 1976 PA1 U.S. Pat. No. 4,086,769--Smith--May 2, 1978 PA1 U.S. Pat. No. 4,087,971--Hart--May 9, 1978
The following publications also describe such devices.
Science Magazine, Vol. 191, Mar. 5, 1976, pages 934-6.
Intersociety Energy Conversion Conference, 1975 record, pages 530-4.
Scientific American, Apr. 1971, pages 118-122.
A heat engine somewhat similar to that described herein is disclosed in Acta Metallurgica, Vol. 25, page 1320. In this device an inch of leaf-spring, an inch of NiTi wire, a three-inch rod, another inch of NiTi wire and another inch of leaf-spring in the same plane as the first are joined in a straight line. This assembly is bent into an S-shape and the outer ends of the leaf-springs are fixed into slots in two pieces of metal mounted four inches apart on an axle. The axle rests across a dish of hot water. When either NiTi bend dips into the water, it stiffens and straightens a little. This increases the bend of the other and displaces the center of gravity of the whole assembly so that it rocks over and dips the cold NiTi bend into the water in its turn. The assembly rocks at about two strokes per second so long as the water is above the transformation temperature. An excellent recent discussion of shape-memory alloys and uses thereof appears in Scientific American, November 1979, pages 74-82.
Although these and numerous other approaches have been proposed and utilized to provide thermally driven motive power sources, most prior art devices have been so mechanically complicated or grossly inefficient as to be impractical.
It is accordingly the object of the invention to provide a thermal engine which is mechanically simple and operationally feasible for certain applications.
It is another object of the present invention to provide a thermal engine which is capable of carrying a load adequate to perform useful work.
It is still another object of the present invention to provide a thermal engine which is capable of scale-up in power developed by the simple addition of units.